S2 auto-focus speed

I am considering the purchase of a portrait lens, either the Nikon 85mm f1.4 or alternatively the 105mm f2 dc. I would also wish to use it with flash for 3/4 length shots of catwalk fashion in low light situations. How good are these lenses in aquiring and locking focus for this purpose? I have in the past tried using a Tokina 80-200 pro atx f2.8 lens but find it struggles both in speed of af and aquiring focus. Many more misses than hits.
Thanking you for any replies.